There are several methods by which environmental wells are drilled under various subsurface conditions. In many situations, the preferred methods are those that allow for the installation of well casings in the hole during, typically using Hollow Stem Augers (HSA). This type of auger has a hollow, steel stem or shaft with a continuous, spiraled steel flight, welded onto the exterior. A HSA often has a bit with teeth at the drilling tip that disturbs soil material when rotated, whereupon the spiral flight transports the soil cuttings to the surface. Multiple augers can be attached to form an auger column to reach the desired well depth. This method is best suited for, though is not limited to, use in soils that have a tendency to collapse when disturbed.
A sacrificial bottom or knock-out plug can be installed at the bottom of the HSA to keep out most of the soils and/or water that have a tendency to enter through the bottom of the HSA during drilling. The use of a knock-out plug can eliminate the need for a drill rod, which can also be used to keep dirt and water out of the HSA. Augering without a drill rod or knock-out plug can also be done, provided that the soil plug, formed in the bottom of the HSA during drilling, is removed before sampling or before final installation of the well casing. Otherwise, when the auger is reversed and removed from the well, the well casing will come up with the plug. Removing this soil plug from the HSA can be accomplished by drilling and washing out the plug using a rotary bit, or augering out the plug with a solid-stem auger bit sized to fit inside the hollow-stem auger. Use of a knock-out plug is often the preferred technique and can be used where no soil sampling is to be conducted during the drilling process.
Typically, a knock-out plug is wedged into the bottom opening of the HSA between the teeth of the bit. When the appropriate depth for the well has been reached, the knock-out plug is literally knocked out, hence the name, of the bottom opening, at depth, usually by pounding it with a drill pipe or the weight of the well casing until the plug breaks. This can be a very time-consuming process, particularly if the bottom plug becomes tightly wedged into the opening, as happens with certain types of soil. Until the knock-out plug is broken and dislodged from of the HSA opening, the remaining process of installing the well is halted.
A typical knock-out plug used in the industry has a frusto-conical shape, similar to a round plate with sloped or angled sides. It is usually a solid piece of material of sufficient rigidity and thickness to be wedged securely into the HSA auger and displace soil as the HSA drill bit moves through the soil. There are also lipped plugs that have generally vertical sides and an enlarged lip around the bottom edge that sits against the outside of the auger to prevent the knock-out plug from going entirely up into the HSA bore. The material utilized for a knock-out plug can depend upon the purpose of the well, but is typically wood or a high-density plastic material, like HDPE or PVC. The problem arises in that a plug material of sufficient strength to withstand the force of soil being compacted and displaced against the outer surface is also usually of sufficient strength to resist being broken or cracked so as to be removed out of the auger.